<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f1()">
<input type="button" value="减" onclick="f2()">
<input type="button" value="乘" onclick="f3()">
<input type="button" value="除" onclick="f4()">
<h3>
  结果：<span>

</span>
  <script>
    let i1 = document.querySelector("#i1");
    let i2 = document.querySelector("#i2");
    let s = document.querySelector("span");
    function f1() {
      if(isNaN(i1.value)||isNaN(i2.value)){
        s.innerText="输入错误！";
        return;
      }
      s.innerText = i1.value*1+i2.value*1;
    }

    function f2() {
      if(isNaN(i1.value)||isNaN(i2.value)){
        s.innerText="输入错误！";
        return;
      }
      s.innerText=i1.value-i2.value
    }

    function f3() {
      if(isNaN(i1.value)||isNaN(i2.value)){
        s.innerText="输入错误！";
        return;
      }
      s.innerText=i1.value*i2.value;

    }

    function f4() {
      if(isNaN(i1.value)||isNaN(i2.value)){
        s.innerText="输入错误！";
        return;
      }
      s.innerText=i1.value/i2.value;

    }



  </script>
</h3>
</body>
</html>